The long term objectives of the proposed research are several. The first is to gain further understanding of the genetic control of the radiosensitivity of human cells and how the genetic changes which accompany neoplastic transformation may effect radiosensitivity. The second is to determine radiation-induced genetic changes associated with the neoplastic transformation of human cells. The third is to obtain quantitative information relating to various aspects of radiation-induced neoplastic transformation of human derived cell lines. The medical significance of the proposed research is principally to the fields of radiation carcinogenesis and radiation therapy. To approach these long term objectives, we will continue our studies with well characterized non-tumorigenic and tumorigenic human cell hybrid lines (HeLa x skin fibroblast). Furthermore, we will initiate new studies with human microcell hybrid lines (HeLa plus a single copy of a human skin fibroblast chromosome, initially chromosome 11 which has been shown to suppress the tumorigenicity of HeLa cells). The rationale for the Specific Aims for the proposed funding period is based on our studies to date. These aims include (1) examining the cell cycle dependency of radiosensitivity and DNA damage induction and repair of both non-tumorigenic and tumorigenic hybrid cells, (2) examining contribution of PLDR and SLDR to cell survival and neoplastic transformation for non-tumorigenic cells treated with fractionated and low dose-rate radiation and (3) isolating and characterizing various properties radiation-induced neoplastic cells (including probing for loss of tumor-suppressor gene). Standard methodologies as well as those either developed or being developed in the laboratories of the P.I. and Co-P.I. will be used. Cell survival will be assessed by colony forming ability, DNA damage by alkali and neutral elution; neoplastic transformation by immunoperoxidase assay of expression of tumor associated antigen, by loss of suppressor gene by RFLP analysis, and by ability of cells to grow tumors when injected into nude mice.